Publications and other reference materials referred to herein, including references cited therein, are incorporated herein by reference in their entirety and are numerically referenced in the following text and respectively grouped in the appended Bibliography which immediately precedes the claims.
There are many situations in which it is required to use an instrument for cleaning out and/or shaping and/or widening a channel that exists in or passes through a solid object. One of the best examples illustrative of the problem and of the shortcomings of the prior art is from the field of dental instruments, specifically root canal procedures. Root canal procedure is one of the most demanding tasks for a dental surgeon. Root canal treatment consists of two stages: (a) shaping, cleaning, and widening of the root canal and (b) obturation of the canal space. The first stage is aimed at removing all tissue remnants, shaping the canal space and disinfecting it, while the second stage is aimed at sealing the root canal and preventing its recontamination. Failure to achieve either of those goals may result in persistent apical periodontitis—associated with periapical bone resorption and occasional or chronic suppuration.
In the simplest case, in which the canal is relatively straight and has a relatively uniform circular cross-section, the work is carried out by starting with a small diameter instrument (known as a dental file) and replacing it with a series of increasingly larger diameter files until the desired final diameter of the canal is attained. If the canal is not straight, using the conventional stainless steel files will most probably result in a poor quality of treatment. In cases in which the curvature is not too great, nickel titanium alloy files are able to adapt themselves to the longitudinal curvature of the canal; thus achieving better results; however existing nickel-titanium alloy files cannot adapt themselves to changes in the shape or diameter of the canal, therefore a series of files must be used to clean out a canal as is the case with stainless steel files. Also the use of any type of existing file in a canal having non uniform or non circular cross-section results in a canal that is either not completely cleaned out or one having a circular cross-section, the uniform diameter of which is at least as large as the maximum width of the original canal.
The anatomy of a tooth is shown in FIG. 1. Topographically tooth 1 is divided into two sections. The crown 2, covered by enamel 4, is the visible part of the tooth. The root 3 is below the gums and anchors the tooth in place in the jaw bone. Within the dentin 6, which comprises the principal mass of the tooth, is located the pulp chamber 5 and one or more root canals 7, both of which are filled with pulp 8.
FIGS. 2A to 2C illustrate the major steps of the root canal procedure. In the first step (shown in FIG. 2A) part of the crown is removed and the pulp is cleaned out of the pulp chamber. In the second step (FIG. 2B) the openings of the root canals are widened. Then (FIG. 2C) the canal system is shaped, cleaned, and widened, removing all pulp particles and infection. In a final step (not shown in the figures) the clean and disinfected root canals are obturated.
Endodontic failures are more common than the profession is willing to acknowledge. The commonly quoted success rates are based mainly on studies carried out in teaching institutes and/or by endodontists. Nevertheless, surveys carried out on the general public indicate that in association with up to 58% of the endodontically treated teeth a periapical lesion was evident (1).
This represents, most probably, a poor quality of treatment (1); nevertheless, it may also represent a basic flaw in the common approach to root canal treatment. Root canals are usually perceived as having a gradually tapering diameter with a round cross section (FIG. 3A). This may be true for many upper anterior teeth; however, it is far from reality in many of the bicuspids and molars. In these teeth ribbon-like root canals, with a flat cross section (FIG. 3B) are common (2). The flat dimension of these canals is at the bucco-lingual plane, which is not seen on a radiograph, and usually has its maximal width at 5 mm from the apex. This anatomy is especially pronounced in the second upper bicuspids, in distal roots of lower molars, as well as in many canines and lower incisors. Additionally, canals with a tear-shaped cross section (FIG. 3C) are common in roots that have two canals, such as the mesial roots of lower molars and those of upper first bicuspids as well as in many of the mesio-buccal roots of upper molars. The fact that all of these anatomical variations are not seen on a regular periapical radiograph is one of the reasons for the common misconception and presents a major obstacle in achieving the goal of high quality endodontic treatment.
The traditional, common, approach for cleaning, widening, and shaping root canals utilizes hand files having round cross-sections and a standardized cylindro-conic shape. Sequential use of files of increasing diameter, with intermitted flushing with antiseptic solution, results in a canal in which a cylindro-conic gutta-pecha cone may intimately be fitted in the apical part of the canal and used as a “master cone”. With the proper paste type sealer and accessory cones they are expected to seal the apical part of the canal. The common cleaning, widening, and shaping procedures are designed to adapt the apical part of the root canals to the form of these standardized master cones.
This common concept is based on the image of a root canal that has a round cross section, which may gradually be enlarged by round files to the shape of a standardized master cone. Although suitable for anterior teeth, when applied in many posterior root canals, this approach may often lead to failure. Cleaning, widening, and shaping a flat root canal with round files will frequently leave a remaining buccal and/or lingual recesses along the root canal filling, untouched and full of tissue remnants, bacteria, or both (3,4). This will inevitably result in failure. Recent studies indicate that this problem is much more common than previously appreciated (2, 3).
When using traditional hand-held stainless steel files some of these recesses are inevitably only partially cleaned. In FIG. 4 is shown a set of stainless steel K-files. A great deal of experience is necessary to choose the proper length and diameter as the canal is cleaned out from the top to the bottom. When the root canal curves it is only possible to follow the curvature by increasing the diameter of the canal. This problem is illustrated in FIG. 5A. If the curvature of the canal becomes to great, generally near the apex of the canal, then one of two undesirable situations arises. The first, shown in FIG. 5B, is that the process is terminated resulting in creation of a ledge in the canal and debris in the untreated lower part of the canal. The second alternative, shown in FIG. 5C, is that the procedure continues until perforation of the tooth occurs, also resulting in a false canal or apex enlargement. One other major difficulty encountered during root canal treatment is shown in FIG. 5D; if one of the solid files breaks and can not be removed, a not infrequent occurrence, then the root canal treatment fails and the tooth is usually removed.
The hand-held stainless steel instruments are currently gradually being replaced by nickel titanium rotary files. As in the case of K-files, current leading nickel titanium rotary files are used as series of instruments with gradually enlarging diameters but, unlike the stainless steel files, are able to adapt longitudinally to the canal shape. Nickel titanium rotary files are more efficient and allow the operator to complete a case in a somewhat shorter time. Unfortunately, this efficiency has a price: the prepared canal is of a perfect round cross section and the cleaning of the recesses in flat and tear-shaped canals is not done.
Recognition of the above problem has led some professional authorities to consider a further enlargement of the apical part of the canals, to a diameter that will include the flat remaining parts. The preparation of a root canal by enlargement for root canals having round, flat, and tear-shaped cross-sections is shown schematically in FIGS. 6A, 6B, and 6C respectively. Such enlargement could not be safely done in bent root canals with traditional stainless steel instruments because of their rigidity (3). Rotary nickel titanium files made it possible to enlarge bent canals in a mesial root of a lower molar to accept a No. 45-50 gutta-percha master cone. Nevertheless, this approach has two short comings: it may frequently lead to either unwanted excessive local thinning of the remaining root canal wall (2) or to the use of a single-cone root canal filling in the apical third of the canal. The first may lead to a higher frequency of vertical root fractures, while the second was discarded long ago for its inferior sealing abilities. Furthermore, in many cases the diameter required to really include the recesses is so large that it will cause a perforation of the canal wall (2) (see FIG. 6B and FIG. 6C).
The issue of cleaning, widening, shaping, and obturation of root canals with non-round cross sections does not currently have an adequate, efficient solution.
It is therefore a purpose of the current invention to provide an instrument that is capable of changing its interior volume, contour, or both during use in cleaning and/or shaping and/or widening a channel, existing in or through a solid object, in order to shape itself to the channel's three-dimensional contour.
It is another purpose of the current invention to provide an endodontic file that is capable of cleaning and shaping and widening root canals having circular and/or non-circular cross sections.
It is another purpose of the current invention to provide an endodontic file whose contour varies during use thus allowing the use of a single file for an entire root canal procedure.
It is yet another purpose of the current invention to provide an endodontic instrument that is sufficiently flexible to allow it to adapt itself longitudinally to the curvature of the root canal up to its apical end.
It is still another purpose of the current invention to provide an endodontic instrument through which antiseptic solution can continuously flow into the root canal while working.
It is a further purpose of the current invention to provide an endodontic file that has significantly greater endurance and strength than existing instruments used to perform root canal treatments.
It is another further purpose of the current invention to provide an endodontic file that requires significantly less time to perform root canal treatments than existing instruments.
It is a still further purpose of the current invention to provide an endodontic instrument that is significantly easier to use and has a significantly shorter learning curve than existing instruments used to perform root canal treatments.
Further purposes and advantages of this invention will appear as the description proceeds.